In a wireless communication system such as wireless LAN, a user is not always transmitting/receiving signals at a location with a constant reception level and the level of a signal received varies from one user to another, for example, the distance from a transmission antenna may be long or may be short. Therefore, suppressing the level of a received signal to within a dynamic range of an analog/digital converter is a technology essential to a wireless communication system. A demodulator performs this operation through an auto gain control (hereinafter referred to as “AGC”) circuit. In order to adjust the amplitude of a received signal to within a dynamic range, the AGC circuit sends an AGC preamble signal before a signal such as a start symbol at the start of a packet and the AGC circuit controls an amplification gain based on the reception level of this preamble signal. The AGC preamble signal is designed to be sent by being inserted in a transmission signal together with data. An AGC error is an error between a target value of the reception level after AGC and actual reception level.
However, in a conventional transmission apparatus and gain control method, when appropriate AGC preambles cannot be set when, for example, the channel quality is poor, an AGC pull-in characteristic deteriorates, the signal may be clipped or the reception level may decrease, and quantization errors may increase, making it difficult to demodulate the received signal without errors. Furthermore, since AGC preambles are not data that is sent to the party on the other end, there is a problem that inserting AGC preambles deteriorates data transmission efficiency.